Production of alkali metals



Patented Feb. 21, 1939 UNITED STATES 2.148.404 PRODUCTION or ALKALIMETALS' Harvey N. Gilbert, Niagara Falls, N. Y., assignor to E. I. duPont de Nemours & Gompany, Wilmington, Del., a corporation of DelawareNo Drawing. Application March 11, 1936,

Serial No. 68,275

. Claims. 204--21) This invention relates to the electrolysis of alkalimetal compounds to produce alkali metals and other valuable products andmore particularly to electrolysis processes which utilize liquid 5 metalelectrodes.

Various methods 'for electrolysis of alkali metal compounds either inthe fused state or in aqueous'solutions with the use of circulatingliquid metal electrodes are well known. An example of this is thewell-known Castner cell used for the production of caustic "soda,wherein mercury serves as the cathode for the electrolysis of aqueoussodium chloride solution. In processes of this nature the alkali metalreleased at the liquid metal cathode alloys therewith and the liquidalloy then is circulated either to react chemically with water or othersubstances to produce caustic soda orother sodium compounds.

A number of methods have been proposed for producing metallic sodium orsodium compounds by the use of a two-cell system employing a'liquidmetal electrode whereby a fused sodium salt is first electrolyzed with aliquid metal cathode to form the sodium alloy and this liquid sodiumalloy then-is used'as anode in a cell employing a fused sodium compoundas electrolyte, e. g., caus- 'tic soda, whereby metallic sodium isreleased at the cathode. This method of producing sodium has notenjoyedany substantial commercial use and so far as I am aware is notcommercially,

used at the present time. A significant disadvantage of this two-cellsystem of producing sodium has been the excessively high temperatures.These high temperatures, e. g., 350 :0. 5 or higher makit impractical toemploy mercury as liquid electrode because of the high vapor pressureof'mercury at such elevated temperatures. Further disadvantages of thehigh temperatures are that thworking life of the equip- 40 ment isseriously shortened because of the tendency of rapid corrosion at hightemperatures, relatively massive and expensive equipment is required,and there is considerable loss of energy in the form of radiated heat.

50 in a non-aqueous secondary cell which may be" operated at a lowtemperature. In such arrangement, mercury (or alkali metal amalgam)would be a suitable metal to serve as the liquid electrode. So far as Iam aware, no one heretofore 55 has prepared a commercially feasiblemethod of doing this. It has been proposed to electrolyze aqueous saltsolution with a mercury. cathode and employ a. second cell having aliquid ammonia solution of salt as electrolyte, where the amalgamresulting from the first electrolysis 5 7 serves as anode. However, thismethod has not proved practicable for commercial operation, partlybecause of the difliculties attending the use of liquid ammonia solutionas electrolyte.

An object of the present invention is to pro- 10 vide a. practicablemethod whereby an aqueous solution of an alkali metal compound may beelectrolyzed witha liquid metal cathode to produce a liquid alkali metalalloy and that alloy be utilized as anode in an electrolytic operationto 15 produce alkali metal, e. g., metallic sodium. A

further object is to provide a commercially feasible method forproducing sodium metal or other alkali metal) by electrolysis utilizingan v alkali metal amalgam as anode attemperatures 20 at which the vaporpressure of mercury is rela; tively low. Other objects will behereinafter apparent. I The above objects are accomplished in accordancewith the present invention by producing 25 alkali metal amalgam byelectrolysis of'an aqueous'alkali metal compound solution with a mercury(or amalgam) cathode and using the amalproduction of sodium) has amelting point of 40 about 225 C. and this mixture may be fused andutilized as molten electrolyte at temperatures of for example 230-250 C.At such relatively low temperatures, the vapor pressure of mercury isless than 1/10 atmosphere.

In one method of practicing the present invention for. the production ofsodium which is given by way of example, an,aqueous solution of sodiumchloride is electrolyzed in a cell having a flowing stream of mercury orsodium amalgam as cathode. As a result of this electrolysis, chlorine isliberated at the anode and means are provided for removing the gaseouschlorine as formed. Sodium liberated at the cathode unites with themercury to form amalgam, so .that'the liquid cathode flowing from thecell is richer in sodium than the mercury or amalgam entering the cell.The concentration of sodium in the amalgam leaving the cold cell mayvary widely; aconcentration of around 0.1 to 0.2% by weight is suitable.The liquid amalgam leaving the cell is led to a second cell in which theamalgam is utilized as anode. Preferably, the amalgam flows constantlythrough the second cell. The second cell employs as electrolyte thefused, eutectic mixture of sodium hydroxide and sodium iodide whichcontains about 55% by weight of sodium hydroxide and 45% by weight ofsodium iodide and has a melting point of about 225 C. The temperature ofthis fused electrolyte is maintained at about 240-250" C. The cathode inthis second cell may be made of iron or steel or other suitableconductive material. The electrolysis in the second cell results inconstant removal of sodium from the flowing amalgam anode and theliberation of molten sodium metal at the cathode. The molten sodium,having a lower specific gravity than that of the electrolyte, risestowards the surface and suitable collecting means are provided above thecathode for entrapping the rising sodium and conducting it out to aclosed receiver so that it is kept out of contact from the air. Fromtime to time the collected sodium may be removed from the receiver.

In addition to the above described eutectic mixture of sodium hydroxideand sodium iodide,

- other'mixtures of these two compounds and varivous other mixtures ofalkali metal hydroxides corresponding hydroxides and halides preferablyhaving the proportions of the alkali metals in the electrolyte at leastroughly equivalent to the proportions in which these metals occur in the,alkali metal amalgam anode.

The selection of the halide to be mixed with the hydroxide and theproportions thereof will be determined chiefly by the temperature atwhich it is desired to operate the electrolytic cell. When using amercury base anode, I prefer to employ an electrolyte which has amelting point not greater than about 300 C. with such ,anelectrolyte-the cell may be operated at temperatures slightly above 300C., e. g. up to 310 6., at which temperatures the vapor pressure of-mercury is around one-third of an atmosphere and hence the loss inmercury due to vaporizatlon at ordinary pressures is small. .11 themelting point of the electrolyte is not over 250 C. the

f'eell may be operated at temperatures sufliciently "low that the vaporpressure of the mercury is not over about one tenth of an atmosphere,under which conditions there is little or no loss of mercury due .tovaporization. Obviously vari- -ous fused mixtures containing an alkalimetal fer to usesubstantially eutectic mixtures of these substances.

lnladditloh to the eutectic mixture 0), hydroxide and sodium iodidewhich has been mentioned above, the eutectic mixtures shown in thefollowing table illustrate thevariety of compositions which may beutilized to produce electrolytes having relatively low melting points:

Eutectic mixture: Melting point C.) NaOH ('79 molar %)+NaBr 260 KOH (72molar%)+KI 250' KQH (65 molar %)+KBr 300 LiOH (45.5 molar %)+LiI 180LiOH (45 molar +LiBr 2'75 LiOH (63 molar %)+LiCl 290 In its broaderaspects; my invention relates to the electrolysis of alow-meltingmixture of alkali metal hydroxide and alkali metal halide, utilizing aliquid alkali metal alloy as anode, so as to liberate alkali metal atthe cathode. In order to obtain a sufficiently low melting point, themixture of alkali metal hydroxide and halide may be an eutectic mixtureor some mixture approximating or approaching the eutectic, i. e., amixture having a melting point lower than that of its lowest meltingcomponent. The invention is not restricted to using alkali metal amalgamas the anode, since other low melting alloys of alkali metals areutilized for this purpose. Examples of such are "Woods metal" and othersimilar alloys of bismuth, lead, tin and/or cadmium. It is preferable,however, in most cases to use an amalgam electrode.

In carrying out the herein described process in a continuous manner overextended periods of time to produce alkali metals 1' have found thatsubstantially no decomposition of the fusedelectrolyte occurs. Thecomposition of the electrolyte remains substantially unchanged overextended periods of time and there is no evidence of anodic oxidation ofthe halide. For example,

in a bath containing iodide, there is no formation of either iodate orper iodine. Hence the cell may be operated practically indefinitelywithout changing or replacing the electrolyte or readjusting thecomposition thereof.

In practicing the present invention, I prefer to utilize-the methodand-i paratusdisclosed and claimed in my copending application flled ofeven date herewith, whereby the liquid metal electrode is applied on thecell in the form of a thin film on a moving metal surface. In thepreferred .form of this method as disclosed in the aforesaid copendingapplication the liquid alkali metal amalgam anode is flowed through-areservoir in the lower portion of the cell while the fused electrolytefloats on the surface of the amalgam in' the reservoir. A disc rotatingon its horizontal axis or other suitable moving surface is arrangedpartly immersed in the reservoir of amalgam and partly projecting intothe electrolyte. By this means the disc is coated with a film of theamalgam and the rotation of the disc constantly brings a thin film ofthe amalgam from the reservoir into contact with the electrolyte, re-.

turning a thin film of amalgam from which a portion of the alkali metalhas been removed by electrolytic action. 1

I claim: 1. A process for the recovery of an alkali metal from an alloyof said metal having a melting point not higher than about C. comprisingemploying said alloy in the liquid state as anode in an electrolyticcell which contains as electrolyte a fused mixture of alkali metalhydroxide and alkali metal halide having a melting point below 300 C.

2. A process for the recovery of an alkali metal from an alkali metalamalgam comprising'employing said amalgam as anode in an electrolyticcell which contains asvelectrolyte a fused mixture of alkali metalhydroxide and alkali metal halide, said mixture having a melting pointbelow 3 00 C.

3. A process for the recovery of an alkali metal from an alkali metalamalgam comprising employing said amalgam as anode in an electrolyticcell which contains as electrolyte a fused, substantially eutecticmixture 01. an alkali metal hydroxide and an alkali metal halide.

4. A process for the recovery of sodium from sodium amalgam comprisingemploying said amalgam as anode in an electrolytic, cell which containsas electrolyte a fused eutectic mixture of sodium hydroxide and sodiumiodide,

- HARVEY N. GILBERT.

